The present invention relates to a system for detecting the presence of an electrically conductive object, the system comprising a first and a second contact intended to be short-circuited by said object, and further comprising a control device for the electric potentials of the contacts.
Such a system is described in the article "Automatic Module Detection" by mr. S. Meguro, published in the IBM Technical Disclosure Bulletin, Vol. 22, No. 10 of March 1980. This system makes it possible to detect the presence of a module in its receptacle, the module having two contacts which serve for short-circuiting two terminals in the receptacle, one coupled to logic 1 level at a high impedance and to a control logic, and the other one coupled to logic 0 level, to signal the presence of the module in its receptacle to the control logic when the two terminals are short-circuited by the contacts of the module.
Such a device is not suitable for detecting the presence of a chip card in its reader. As a matter of fact, the number of contacts on an object of this type is limited and it is not desirable to use two of these contacts exclusively for detection purposes. It is more advantageous to use one of the contacts which serve for the power supply of the integrated circuit on the card, preferably its positive supply terminal. However, the potential applied to this terminal is not constant in time. In fact, insertion of the card while the supply voltage is not zero could damage the integrated circuit. Therefore, care must be taken that the potential of those contacts of the detection system which serve for the power supply of the integrated circuit is zero in an initial stage. When the card is inserted correctly into its receptacle its presence is detected and the supply voltage should not be applied to the positive supply terminal of the integrated circuit until after this, to start the actual operating stage. The detection system should therefore be capable of operating on two different supply potentials.